This 'true' for allow-null is not correct. (It was wrong in 7.2 as well.) If you pass a null path into FileTimerPersistence, in start() it will fail in the call to

pathManager.getValue().resolveRelativePathEntry(path, pathRelativeTo)

I guess it will "work" if pathRelativeTo != null, resulting in a path pathRelativeTo/null but it will fail if pathRelativeTo is null.